The present invention relates to a method for fabricating dielectric capacitors, and precisely to a method for fabricating dielectric capacitors usable in ferroelectric nonvolatile memories, etc.
Ferroelectric memories are fast-reloadable nonvolatile ones for which are used high-speed polarization inversion and remanence of ferroelectric thin films. In conventional ferroelectric memories, transistors and ferroelectric capacitors were aligned in the plane direction of a substrate. The ferroelectric capacitor is formed by laminating, on a silicon oxide film, a buffer layer of a titanium metal film having a thickness of 30 nm or so, a lower electrode of, for example, a platinum film having a thickness of 200 nm or so, a ferroelectric film of a PZT [Pb(Zr,Ti)O.sub.3 ] film or SBT (SrBi.sub.2 Ta.sub.2 O.sub.3) film having a thickness of 200 nm or so, and an upper electrode of a platinum film having a thickness of 200 nm or so, in that order.
To increase the information recording density in those memories, transistors and ferroelectric capacitors must be disposed on a substrate in layers, for which the capacitors are so-called stack-type capacitors. In that structure, each transistor is electrically connected with the lower electrode of each ferroelectric capacitor via a silicon plug.
However, reactive ion etching (RIE) of platinum is difficult, and fine working to give capacitors having a lower electrode of platinum is difficult.
Forming a ferroelectric film in capacitor fabrication generally requires heat treatment at high temperatures falling between 600.degree. C. and 800.degree. C., which, however, involves thermal diffusion between a silicon plug and the lower electrode that underlies the film whereby silicon having diffused into the lower electrode is oxidized in the upper layer of the lower electrode to retard the electroconductivity of the lower electrode, or, as the case may be, silicon further diffuses into the ferroelectric film to significantly worsen the capacitor characteristics.
On the other hand, fabricating ordinary MOS memory structures requires forming gas annealing at temperatures falling between 400.degree. C. and 500.degree. C. for transistor recovery, prior to metal wiring operation. However, it is known that such forming gas annealing worsens the characteristics of ferroelectrics such as SBT, etc. In addition, where the lower electrode in capacitors is made of a noble metal except platinum and gold, the noble metal is oxidized through heat treatment for dielectrics. Forming gas annealing after fabrication of ferroelectric capacitors having a lower electrode of a noble metal except platinum and gold much more worsens the dielectric characteristics of the capacitors than those of capacitors where the electrode is made of platinum or the like that is not oxidized at all. This is because the noble metal oxide having formed through the heat treatment for dielectric formation is reduced through the forming gas annealing, thereby bringing about a significant change in the capacitor structure.